Injection molded devices having hollow interior portions or defining interior spaces are often formed using molding equipment having a hollow outer portion and an inner core portion. A molding space between the outer portion, inner core, and possible additional mold portions is injected with a molding material, such as a heated thermoplastic material. The molding equipment, and in turn the molding material, is then cooled to a temperature below the glass transition temperature of the molding material and the finished product formed from the molding material is removed from the molding space. The molding space is typically formed with draft angles to permit removal of the inner core portion from the finished product and/or removal of the finished product from the hollow outer portion. Examples of devices formed in this manner would be syringe barrels and plungers, which typically have generally cylindrical bodies defining interior regions. The interior region and outer surface of the cylindrical body are formed with draft angles, such these surfaces are not parallel to a central axis of the body, but are instead slightly angled with respect thereto to form a generally conical shape.
A stage of a molding operation of a plastic syringe barrel 12 having this configuration is shown in FIG. 1A. As shown, the outer surface 48 of the mold inner core 36 and inner surfaces 33, 35 of lower plate 32 and upper plate 34 are disposed at a draft angle θ (theta) with respect to central axis x of the molding equipment 30. This permits withdrawal of inner core 36 in direction D from the interior of the finished syringe barrel 12 without interference between the inner surface 15 of syringe barrel body 14 and outer surface 48 of inner core 36. FIG. 1A shows the assembly during withdrawal of the inner core 36 from the syringe barrel 12. It should be noted that for illustrative purposes the angle θ is somewhat exaggerated in FIG. 1A with respect to that of a typical syringe barrel.
A stage of a molding operation of a plastic syringe plunger 124 is shown in FIG. 1B. As shown, the outer surface 348 of the mold inner core 54 and inner surfaces 51, 53 of lower plate 50 and upper plate 52 are disposed at a draft angle φ (phi) with respect to central axis x of the molding equipment 49. This permits withdrawal of the molded syringe plunger 124 in direction D from the molding space 55 defined in lower plate 50 without interference between the outer surface 323 of plunger body 125 and inner surface 51 of lower plate 50. FIG. 1B shows the assembly during withdrawal of the plunger 324 from the molding space 55. It should be noted that for illustrative purposes the angle φ is somewhat exaggerated in FIG. 1B with respect to that of a typical syringe plunger.
After the syringe barrels such as 12, or any other syringe barrels or other vessels described herein, are molded, it is frequently desirable to provide them with an SiOx barrier layer and/or an SiwOxCy lubricity or hydrophobicity or other surface property modifying layer as extensively explained, for example, in U.S. Publ. Appl. No. 2010/0298738 A1, published Nov. 25, 2010, issued as U.S. Pat. No. 7,985,188 on Jul. 26, 2011. The latter publication and patent are incorporated here by reference to show suitable barrier, lubricity, and surface modifying layers and how they can be applied.
After molding, a finished product is assembled including a syringe plunger used to force a liquid dosage out of the syringe for administration into a patient. The plunger is slidably disposed within cylindrical body of the barrel. The plunger ideally has approximately the same outer diameter as the inner diameter of the syringe barrel, in order to permit slidable engagement therewith while preventing leakage of the liquid dosage from gaps between the plunger and barrel. The draft angles of typical plastic syringe barrels and plungers, typically about 1° to 3° can create difficulty in this respect, as they cause variations in the inner diameter of the barrel and/or plunger. Several measures can be taken to compensate for this. For example, the plunger may be formed of an elastomeric material that permits deformation thereof during sliding within the barrel cylindrical body. The outer diameter of the elastomeric plunger is large enough to compensate for the variation in the inside diameter of the syringe barrel. The oversized plunger creates interference with the syringe barrel that requires higher force to move plunger within the syringe barrel. One measure taken to address the higher plunger force required with a syringe having an elastomeric plunger the application of a lubricity layer, such as silicon oil applied to the interior of the syringe barrel and/or the plunger to lubricate and facilitate sliding of the plunger within the barrel. FIG. 10 illustrates one of the problems with syringes employing this type of lubricity layer. As shown, the layer material, which is silicon oil in the example of FIG. 10, can be displaced by the plunger. Over time and/or due to sliding of the plunger, portions of the silicon oil typically migrate, leading to nonuniformity of the layer. This can make subsequent sliding of the plunger more difficult. Further, some of the lubricity material can expelled from the syringe along with the dosage, and in some cases injected into a patient receiving the dosage. For this reason, plastic molded syringes are often intended to be used only once and disposed of.
Another problem caused by the inclusion of draft angles within a plastic syringe barrel and/or plunger is that of nonuniformity of the pressure required to be applied to the barrel during dosage administration. Due to the decreasing inner diameter of the barrel wall and/or the increasing outer diameter of the plunger wall, the amount of pressure applied must be increased as the plunger approaches the needle end of the syringe. This can cause stalling during administration, which can result in pain to the patient receiving the dosage. Additionally, this may cause difficulty in administering a dosage using an autoinjector, i.e., a mechanical device that administers a dosage using, for example, a spring loaded mechanism or motor, as these devices may not be able to perceive a change in resistance as readily as a human administrator.
Other prior patents in this area are U.S. Pat. Nos. 5,141,430; 5,022,563; and 5,971,722.
Glass syringes and other vessels have traditionally been favored over thermoplastic syringes and vessels because glass is more gas tight and inert to pre-filled contents than untreated plastics. Also, due to its traditional use, glass is well accepted, as it is known to be relatively innocuous when contacted with medical samples or pharmaceutical preparations and the like. Glass syringes are also fabricated from extruded tubing, which does not require a draft angle. But it is desirable for certain applications to move away from glass vessels, which can break and are expensive to manufacture, in favor of plastic vessels which are rarely broken in normal use (and if broken do not form sharp shards from remnants of the vessel, like a glass tube would) and inexpensive to manufacture by injection molding in a multi-use mold. A need exists for a plastic syringe that can be formed by injection molding in a multi-use mold free or partially free of draft angles, in order to eliminate the problems discussed above.